Direct Access Storage Devices (DASD) are a class of disk data storage devices currently used in data processing environments. Typically, the burst data rate between the DASD and its associated electronic circuits, such as a cache memory, varies from less than two megabytes per second through six or so megabytes per second. Because of such high burst rates and the electronic speeds of circuits attached to the DASD, it has been common practice to synchronize the operation of a host computer access to a DASD to the rotation disk being accessed. This mode of operation is called a synchronous mode, i.e., the operation of the data transfer is synchronized to the rotation of the disk in the DASD. Such synchronous operations are satisfactory up to a predetermined lengths of signal cables extending between the DASD and its controller and connecting host processors. The reason for the limitation in the spacing is propagation time of the signals between the host processor and the DASD controller. Such propagation time, if extended beyond a predetermined maximal elapsed time, requires a longer time to connect and reconnect to a host processor and exchange control signals then the DASD uses to scan a gap between a control field and a data field on the disk surface. If the propagation time exceeds the gap scanning time, then an additional rotation of the DASD disk is required for reading the data field after scanning a control field. That is, in CKD architecture (count, key, data) the count and key field always precedes the data field for each record. Except in a FORMAT WRITE operation, the count field is in a read mode while the data field can be either in the read or write mode. Using the CKD architecture, it is required to read the count field, scan a gap, and then read the data field. If the gap scanning time is less than the control signal propagation time, such synchronous operations are impossible. The above discussion does not describe all possibilities, the discussion is intended to explore a need for controlling and accessing DASD in other than a synchronous accessing mode.
With the advent of optical fiber channels having a data burst rate much greater than the data burst rate of the DASD, such as four times the burst rate. The data rate is combined with extra long cable length, such that a propagation delay is greater than the gap scanning time of the DASD, the rearrangement creates performance problems for a DASD peripheral subsystem. It is also desired to maximize utilization of the channel, i.e., maximize utilization of the optical fiber channel such that once a burst of data and its control signals are transferred over the channel, such transfer proceeds for as long as possible. Rate changing buffers can accommodate some of the burst rate differences; however, more control is needed for efficiently using a DASD with such a fiber optic channel that has long propagation delays. It is desired to operate the DASD and the channel in a non-sychronous mode such that operations with both the channel and the device can be optimized to a maximal extent for the data transfer being effected between the host processor.